Process of impregnating an instantaneously wettable paper with a rubber latex



I. R. DUNLAP PROCESS OF IMPREGNATING AN INSTANTANEOUSLY Get. 3, 1967 WETTABLE PAPER WITH A RUBBER LATEX Filed June 6 1965 INVENTOR.

BY d M ATTORNEY United States Patent 3,345,204 PROCESS OF IMPREGNATING AN INSTANTA- NEOUSLY WE'I'IABLE PAPER WITH A RUB- BER LATEX Isaac R. Dunlap, Cranbury, N.J., assignor to Johnson & Johnson, a corporation of New Jersey Filed June 6, 1963, Ser. No. 286,065 3 Claims. (Cl. 117-102) The present invention relates to the manufacture of unified mechanically altered papers, more particularly to the impregnation of saturating papers and the like with an aqueous latex to produce unified papers suitable for use as backings for pressure-sensitive adhesive tapes, sandpaper, and the like, and in artificial leathers and similar products. When the term mechanically altered is used herein, it means creped, or compacted in accordance with the teachings of Dunlap United States Patent No. 3,055,496.

These unified papers possess a number of properties which vary somewhat in importance depending upon their specific use. For instance, for pressure-sensitive ad hesive masking tape backings, the paper must possess high elongation, good flexibility and conformability, on the one hand, and delamination resistance, good tensile strength, both wet and dry, and resistance to tear, on the other hand. For sandpaper backings and artificial leathers some of the same properties are important and, in addition, the paper must be relatively free of fiber picking and must possess characteristics which allow it to be flexed without blistering or cracking. The paper is said to be unified when its fibers are held together and it possesses the desired resistance to delamination, fiber picking, blistering, tearing, and the like. In the case of masking tapes, delamination resistance is necessary to allow the tape to be unwound from a roll without splitting or delamination of the backing into plies or layers because of the adherence of the outside of the backing to the adhesive on the inside of the tape. This same delamination resistance also is important to allow masking tape to be removed from a painted surface after it has served its intended function.

The most common method of unifying the paper is to incorporate an aqueous unifying agent, common- -ly termed an impregnant, in the paper to bond the fibers together. Various extensible polymeric materials such as rubbery polymers and mixtures thereof have been used for this purpose. Such impregnants allow the sheet to retain a major portion of the characteristic flexibility of a paper web and yet bind the fibers against delamination of the backing when a roll of pressure-sensitive tape is unwound, for example. Normally, suflicient unification to prevent delamination can be attained if it is possible to incorporate sufficient impregnant in the sheet. Typically, the paper is saturated with the impregnant to provide the necessary unification and the solids content of the impregnant is regulated to control the amount of impregnant solids which are deposited in the sheet.

It is important in papers of this type that a substantial amount, say about 20150%, of the impregnant solids be substantially uniformly distributed throughout the area and thickness of the paper so that the fibers of the paper are bonded to one another by the impregnant, and the paper, therefore, is unified. Conventionally, to ac complish this the starting paper literally has been submerged in a pool of impregnant for a substantial period of time to insure that the impregnant fully penetrates the sheet. The most commonly used technique involves floating the starting paper on the surface of an impregnant bath and then causing it to dip down and become submerged in the bath for a period of time, apparently on- Patented Oct. 3, 1967 the theory that during the flotation stage the paper becomes wet and the air contained in its pores is driven ofi so that when it is submerged the impregnant can penetrate easily into all of the pores of the paper and uniformly distribute itself therein. After submersion, the paper is equeezed in the nip between a pair of squeeze rollers to remove the excess latex and achieve the desired level of impregnation.

There are several major disadvantages to this conventional impregnating technique. When the starting paper is mechanically altered as by creping, or compacting, in accordance with the teachings of Dunlap Patent No. 3,055,496, to improve elongation, flexibility and delamination resistance, and then treated in the above described manner; a certain amount of the creping or compaction will be removed or squeezed out during impregnation and passage between the squeeze rollers. In the case of a creped starting paper, a relatively great amount of crepe normally is squeezed out during this process. As a result, less elongation is available in the finished product. Secondly, since the starting paper is thoroughly wet and,

in fact, submerged in the aqueous impregnant for a pe-,

of squeeze out at higher speeds.

This invention is concerned with papers which are instantaneously wettable by water, i.e., wettable through the thickness of the paper in a fraction of a second. For instance, if a typical creped saturating paper is wet on one side only, it will wet through instantaneously and will not curl, whereas a comparable coating stock will curl because it swells adjacent the wet surface before it wets through. I discovered that an instantaneously wettable starting paper of this type wets through in a surprisingly small fraction of a second and that if oneside of the paper is exposed to an excess of latex for an extremely short period of time such as about 0.01 or 0.02

second, and the excess is removed by squeezing the paper in the nip between a pair of squeeze rollers at the end of this short period of time; sufficient latex is picked up by the paper to impregnate it to the desired level. Any amount of latex solids from about 20 to by weight of the dry fibers may be substantially uniformly distributed throughout the area and thickness of the sheet by exposing the starting paper to an excess of latex in this manner for only this short period of time. Thus, I discovered that the flotation period previously thought necessary to remove the air from the starting paper, and the submersion of the paper in the latex for the period of time previously thought necessary to distribute the latex thoroughly throughout the paper, both are completely unnecessary.

I was first able to measure in the laboratory the time required for the starting paper to pick up the desired amount of latex by modifiying a technique which only recently has been developed for measuring penetration of a paper sheet by coating color components. This tech-' nique is described by Hemstock and Swanson in TAPPI, vol. 40, No. 10, October 1957, pp. 833-838, which discloses its use for measuring the quantity of a paper coating composition absorbed into a sized coating base sheet of paper. Equipment of the type described by Hemstock and Swanson in this article may be used to expose one side of a piece of creped saturating paper wrapped around.

as deposited upon an inclined glass surface. The velocity of the roll over the drop may be calculated as explained by Henistock and Swanson and the time that the paper is exposed to the drop is obtained by using the formula where S is the length of the area on the paper wet by the drop and v is the aforesaid velocity. The paper area wet by, or exposed to, the drop, and the weight of latex solids picked up by the paper both are determined and from this percentage pickup of latex solids is calculated. Sections of the impregnated area of the paper may be taken to assure that the latex is distributed substantially uniformly throughout the area and thickness of the sheet. From this it is possible to determine that the desired amount of latex may be picked up in a given time, such as 0.03 second. To decrease the time of exposure it is necessary to increase the velocity of roll along the inclined surface or decrease the size of the drop, if possible, or both. I also used the apparatus shown in FIG. 1 and described hereinafter to determine time required to pick up the desired amount of latex in accordance with the process of this invention.

I further discovered that when a mechanically altered and instantaneously wettable paper is impregnated by exposure to an excess of latex for such a short period of time and squeezed in the foregoing manner at the end of this period, the crepe or compaction is not squeezed out substantially. Whats more, since the paper is impregnated and squeezed while it still retains a substantial amount of its dry strength, special wet strength starting papers are not necessary. In this connection, I have determined that even if this starting paper is exposed to an excess of latex for a longer period of time which, however, is substantially less than that required for the paper to reach its maximum swollen dimension, that is, substantially less than about 0.3 second, and the excess is removed at the end of this time period, neither crepe nor compaction is squeezed out substantially and non-wet-strength starting papers may be used.

In my opinion squeeze-out of crepe or compaction is a function of the softness or plasticity of the paper as it passes through the squeeze rollers. If the paper is softened and plasticized to a high degree, i.e., almost fully softened and plasticized, the mechanical working which occurs during squeezing removes a substantial amount of the crepe or compaction. For instance, in the above-described prior art impregnation techniques, the starting paper was fully softened and plasticized prior to squeezing with the result that serious squeeze-out occurred. In view of my discovery that the desired amount of latex may be picked up by the paper in an extremely short period of time, I concluded that the problem of squeezeout could be solved by squeezing the wet paper to remove the excess of impregnant well before the paper becomes fully softened and plasticized. Since there is no known reliable method of measuring the time taken or required for the paper to become fully softened or plasticized and since it is believed that swelling of the paper occurs at substantially the same rate as softening and plasticizing, the time required for softening and plasticizing can be estimated by reference to the time the paper takes to reach its maximum swollen dimension. Since it also is believed that swelling takes place at a slightly faster rate than softening and plasticizing, this is a safe criterion for the purposes of this invention. Thus, as indicated in the preceding paragraph, it can be stated that if the paper is squeezed to remove the excess of latex substantially be fore it reaches its maximum swollen dimension, no substantial squeeze-out will occur. The swelling referred to is swelling of the sheet as a whole, since swelling of the individual fibers has little effect on swelling of the sheet. Swelling of the sheet may be determined by measuring its increase in length when a strip of starting paper according to this invention is exposed to the aqueous liquid. The

sheet about 20150% of thelatex solids by weight of the dry fibers by the following method. At least one side of the starting sheet is exposed to an excess of latex for a period of time greater than that required to pick up the desired amount of latex and substantially less than that required for the paper to reach its maximum swollen dimension and the excess of latex is removed from the sheet at the end of this time period. Preferably, the excess of latex is removed from the sheet by squeezing the sheet in the nip between a pair of squeeze rollers at the end of said period.

As indicated above, I have determined that for a conventional creped saturating paper of the prior art, the time required for the paper to pick up the latex is about 0.01 or 0.02 second or perhaps even less, and the time required for the paper to reach its maximum swollen dimension is about 0.3 second. Thus, for this paper, the time during which the sheet is exposed to an excess of latex might be between about 0.01 second and 0.2 second; the upper end of the range being substantially less than the swelling time of about 0.3 second. Preferably, however, the excess of latex is removed as early as possible to minimize swelling, crepe pull-out, and loss of strength during the process. Thus, for best results, the time the paper is exposed to the excess of latex normally would be less than about 0.1 second and close to about 0.01 or 0.02 second.

The process of this invention has the following main advantages:

(1) Unified mechanically altered papers having increased elongation may be produced.

(2) The speed of the process may be increased substantially without loss of quality and, if desired, with increased elongation in the resulting product.

(3) Starting papers without special wet-strength treatment may be used with no ill effects.

' As indicated hereinbefore, squeeze-out of crepe or compaction is minimized in the process of this invention thereby assuring a high elongation after squeezing. Normally an impregnated creped paper of the type described is dried and cured after squeezing by passing the impregnated paper under tension over drying drums such as are commonly used in paper manufacture. Both the rate of drying and of curing the impregnant are somewhat dependent upon the tension applied to the sheet. Inevitably a small amount of elongation is pulled out in order to provide the necessary tension and contact with the sheet. In the conventional impregnation process, both squeezeout and pull-out increase at higher speeds. The process of this invention may be operated at much higher speeds with no loss in physical properties. In fact, increased elongation may be attained in combination with increased speed as will be illustrated in Example I, hereinafter.

Other and further advantages of the invention will be apparent from the following description and claims taken together with the drawings wherein:

FIG. 1 is a schematic view of apparatus for carrying out the process of this invention according to a preferred embodiment thereof;

FIG. 2 is a schematic plan view of apparatus for measuring swelling time of a starting paper according to this invention;

FIG. 3 is a schematic end view taken from the left of FIG. 2; and

FIG. 4 is a schematic elevational view of the apparatus of FIG. 2.

Referring to the drawings, there is shown a starting paper 11 which may be a conventional creped saturating paper or some other mechanically altered paper which is instantaneously wettable by water, and a pair of squeeze rollers 12 and 13 for use in impregnating the paper and removing the excess of latex therefrom. The starting paper is led over a guide bar 14 and then under a positioning bar 15 which is vertically adjustable to determine the amount which the paper 11 wraps about the lowermost squeeze roller 13, as will be described more fully below. The squeeze roller 13 is driven to rotate about its axis in a pool 16 of an aqueous latex impregnant. The pool is maintained at a substantially constant level in a trough 17 by automatic filling means, not shown, provided for this purpose. The roller 13 rotates clockwise through the pool and picks up a film of impregnant which it carries on its upwardly moving surface toward the starting paper 11. The starting paper then contacts the impregnant film at approximately the point A on its way through the nip formed at approximately the point B between the rollers 12 and 13. The viscosity and solids content of the impregnant and the speed of the roller 13 through the pool 16 are so adjusted that the film or layer of impregnant carried by the roller 13 to the point A presents more latex incrementally to the paper as it passes between the points A and B than it is necessary for the paper to pick up to reach the desired latex solids content. Thus, the paper 11 is in contact with an excess of latex between the points A and B.

At or before the point B, the paper is squeezed so as to remove the excess of latex therefrom. The time taken by the paper 11 to pass between the points A and B is greater than that required for the paper to pick up the desired amount of latex and substantially less than that required for the paper to reach its maximum swollen dimension. As mentioned above, the amount which the paper 11 wraps about the roller 13 may be regulated by the adjusting bar. The wrap may be increased by lowering the adjusting bar 15 or decreased by raising the adjusting bar. When the speed of the paper through the apparatus of FIG. 1 is maintained substantially constant and the bar 15 is lowered to increase the wrap, the distance between the points A and B, and therefore the time the paper is exposed to an excess of latex, is increased; whereas if the bar 15 is raised and the wrap is decreased, the distance between the points A and B and the time during which the paper is exposed to an excess of latex is decreased. correspondingly, when it is desired to increase the speed of the paper through the apparatus, the bar 15 may be lowered to increase the distance between points A and B and therefore provide the same contact time with an excess of latex. In any case, after passage through the squeeze rollers 12 and 13, an impregnated paper 18 is provided which, after drying, will have the proper latex solids content to provide a unified paper 7,

possessing the desired properties.

FIGS. 2, 3, and 4 illustrate an apparatus and technique for determining the time required for starting paper according to this invention to reach its maximum swollen dimension.

In this apparatus, a strip of mechanically altered and instantaneously wettable paper of this invention is secured in a horizontal position between a pair of clamps 22 and 23 attached to opposite ends of the strip. The clamp 22 is fixed in position and mounted on a conventional stand I 24. The clamp 23 at the other end of the strip 21 is attached to a wire 25 which is led over a rotatable pulley 26 and then downwardly to one end 27 of a marking arm 28. A weight W is also attached to the same end 27 of the marking arm and the arm 28 is mounted for pivotal movement on a shaft 29 which is rotatably mounted in conventional means, not shown. A horizontally extending marking pencil 31 is secured in position at the other end of the arm 28. Thus, the weight tends to move the end 27 of the marking arm downwardly and lift the marking pencil 31 at the opposite end of the arm against the resistance of the wire 25 attached to the strip 21 through the clamp 23.

A sheet of recording paper 32 is carried by a pair of vertical cylinders 33 and 34 and the cylinder 34 is driven clockwise by means not shown in'such a way that the point of the marking pencil 31 contacts the moving paper 32. Thus, when the strip 21 is positioned between the.

clamps 22 and 23, in a dry condition, the above described apparatus will become stabilized so that the pencil 31 remains in a fixed position and draws a first horizontal line on the recording paper 32.

An excess of an aqueous latex is applied to the underside of the paper strip 21 in the form of a film 35positioned on a horizontally disposed piece of plate glass 36 which, in turn, is carried by a table 37 of a laboratory jack. The table is operated to raise the film 35 carried by the plate glass 36 into contact with the strip 21. Since the strip is instantaneously wettable by the latex, it will begin to swell immediately and therefore lengthen under the tension applied by the weight W. As soon as the strip 21 begins to lengthen the marking arm 28 will begin to pivot on the shaft 29 and the marking pencil 31 will begin to lift. The marking pencil 31 will continue to lift during the time that the strip 21 continues to swell and increase. in length. This will cause the pencil 31 to draw a curved line on the recording paper since its upward motion will be combined with the horizontal motion of the paper. However, as soon as the paper reaches its maximum swollen dimension and therefore no longer increases in length, the pencil 31 will again assume a fixed position above its original location and begin to draw a second horizontal line on the recording paper 32. The time required for this particular starting paper to reach its maximum swollen dimension is therefore determined by measuring the time taken for the pencil to move from one horizontal line to the other.

The following examples are included to illustrate various embodiments of the process and of products according to this invention:

Example I A conventional creped kraft saturating paper of the type conventionally used for producing backings for pressure-sensitive adhesive masking tapes and having a basis weight of 28.5 lbs. per ream (24 x 36 x 480) is provided as one starting paper. This paper has a wet strength of about /2 lb./ inch width provided by hot flash wet strength treatment as described in US. Patent 2,116,544, possesses an elongation to break of about 11% and is identified below as paper A. A, second starting paper (paper B) similar to that of paper A but with no wet strength treatment of any kind and with approximately 13 to 14% elongation to break, also is provided.

The impregnant is an aqueous latex comprising essentially a mixture of 50% of a carboxyl modified butadieneacrylonitrile polymer, and 50% of a similarly carboxyl modified butadiene-styrene polymer. The impregnant also comprises minor proportions of phenol formaldehyde and an antioxidant. These materials are prepared in the form of an aqueous latex having a total solids content of approximately 33.6%.

Both papers A and B are impregnated to the levels indicated in the following Table A. Each paper is then impregnated, both (1) by the standard float and dip technique which is conventional in the manufacture of paper backings for masking tape and (2) by the technique of this invention. In each case the apparatus used is operated in such a way as to give maximum possible elongation at the squeeze roll speed indicated. In impregnating the paper according to this invention, apparatus, such as shown in FIG. 1 and described hereinbefore, is used and one side of the paper is exposed to an excess of latex for about 0.02 second before being squeezed in the nip between the rollers. Tensile strength,

7 elongation to break in the machine direction, and the linear squeeze roll speed of'the process are indicated in Table A for each run.

It is apparent from Table A that when papers A and B are impregnated according to this invention, greatly increased elongations and process speeds are attained. In fact, the elongations of both papers after impregnation :are greater than the elongation to break of the papers prior to impregnation. This is an improvement of great importance for many applications. The method of this invention operated 50% faster than the conventional process, i.e., at 90 y.p.m., as compared with 60 y.p.m., yields elongations of 16.9 and 17.5% as compared with 12.9 and 11.8% for the conventional process or increases of about 31 and 48%, respectively. If the two processes are operated at approximately the same speed (85 and 90 y.p.m.), elongation is increased from 8.6 to 16.9% for a percentage increase of about 96.5%.

Example 11 Papers A and B of Example I again are impregnated according to this invention, but this time an attempt is made to reduce the increased elognation of the resulting impregnated paper to a more conventional level by applying increased tension during drying of the paper after impregnation. The following results are attained:

In this example process speeds are increased with no losses in the properties of the resulting papers.

, Example III This example illustrates how elongation is controlled in the process of this invention by increasing or decreasing the ratio of wind up speed to unwind speed and thereby increasing or decreasing the tension during drying of the paper after impregnation. Unwind speed is the speed of the paper as it is unwound from a supply roll, not shown, before the squeeze rollers 12 and 13 in FIG. 1, and wind up speed is the speed of the impregnated paper as it is wound up after passing through or over conventional drying equipment such as a stack of drying cans, also not shown. Variations in percent elongation for papers A and B of Example I for diife'rent ratios of wind upto unwind speed are indicated below.

TABLE 0 Unwind Wind Up Ratio of Wind Elongation, Paper Speed, y.p.m. Speed, y.p.rn. Up Speed to percent;

Unwind Speed A 84. 5 89. 5 1. 058 14. 5 B 84. 0 89. 5 1. 063 12. 0 B 87. 3 89. 5 1. 023 17. 0

Having now described the invention in specific detail and exemplified the manner in which it may be carried into practice, it will be readily apparent to those skilled in the art that innumerable variations, applications, modifications, and extensions of the basic principles involved may be made without departing from its spirit or scope.

The invention claimed is:

1. The process of impregnating a mechanically altered paper sheet which is instantaneously wettable by Water with a rubbery latex so as to distribute substantially uniformly throughout the area and thickness of the sheet about 20150 percent of the latex solids by weight of the dry fibers, which comprises exposing at least one side of said paper to an excess of latex for a period of time of at least about 0.01 second and less than about 0.2 second, and removing the excess of latex at the end of said time period.

2. The process of impregnating a saturating paper according to claim 1, wherein the paper is squeezed in the nip between a pair of squeeze rollers at the end of said time period to remove the excess latex from the paper.

13. The process of impregnating a saturating paper according to claim 3, wherein said time period is less than about 0.1 second.

References Cited UNITED STATES PATENTS 877,777 12/ 1907 Wickel. 1,660,204 2/1928 Moses 117155 X 2,037,130 4/1936 Hill 117163 X 2,060,897 11/1936 vRichardson et al 117155 3,055,496 9/1962 Dunlap 117122 X WILLIAM D. MARTIN, Primary Examiner.

H. W. MYLIUS, M. LUSIGNAN, Assistant Examiners. 

1. THE PROCESS OF IMPREGNATING A MECHANICALLY ALTERED PAPER SHEET WHICH IS INSTANTANEOUSLY WETTABLE BY WATER WITH A RUBBERY LATEX SO AS TO DISTRIBUTE SUBSTANTIALLY UNIFORMLY THROUGHOUT THE AREA AND THICKNESS OF THE SHEET ABOUT 20-150 PERCENT OF THE LATEX SOLIDS BY WEIGHT OF THE DRY FIBERS, WHICH COMPRISES EXPOSING AT LEAST ONE SIDE OF SAID PAPER TO AN EXCESS OF LATEX FOR A PERIOD OF TIME 